Conventionally, to reduce distortion of sound output from a speaker or to suppress overmodulation of broadcast output, a sound signal limiting amplifier that limits sound output to a given value is widely used (refer to Japanese Patent Unexamined Publication No. H08-316760 for example).
A description is made of a configuration of sound signal limiting amplifier 300 described in Japanese Patent Unexamined Publication No. H08-316760 using FIG. 16. An input sound signal to input terminal 310 typically contains all the frequency components from low frequency range to high frequency range. The input sound signal passes through low frequency selector circuit 311a as a low-pass filter that limits the frequency bandwidth to a predetermined frequency. On the other hand, the sound signal similarly passes through high frequency selector circuit 311b as a high-pass filter. The sound signals having passed through low frequency level limiting amplifier circuit 312a and high frequency level limiting amplifier circuit 312b are synthesized by adder circuit 313 and are output to output level limiting amplifier circuit 314.
When a signal level of a signal having passed through low frequency selector circuit 311a does not undergo level limitation by low frequency level limiting amplifier circuit 312a and a signal level of a signal having passed through high frequency selector circuit 311b does not undergo level limitation by high frequency level limiting amplifier circuit 312b, a signal synthesized from the two signals by adder circuit 313 is output with its frequency characteristics remaining flat.
Meanwhile, when a sound signal with its amplitude of low-frequency components larger than that of high-frequency components is input to input terminal 310, the sound signal having passed through low frequency selector circuit 311a undergoes level limitation by low frequency level limiting amplifier circuit 312a. On the other hand, a sound signal having passed through high frequency selector circuit 311b does not undergo level limitation by high frequency level limiting amplifier circuit 312b. Hence, conducting the signals through adder circuit 313 provides output signal h with only its low components level-limited and with its high components not level-limited. However, an adding operation by adder circuit 313 can cause a given reference limitation level to be exceeded. In this case, output signal h is level-limited by output level limiting amplifier circuit 314 and is eventually output as output signal i.
In the meantime, a volume amplitude limiting device is typically designed to specifications such that the level of a sound signal to be input does not exceed the maximum rated value of a speaker or sound amplifier. However, equipment with an actual speaker built-in often produces unpleasant sound due to resonance of the housing of the equipment or sound distortion of the speaker even if output from the speaker does not exceed the maximum rated value. Accordingly, to keep pleasant sound quality, sound output needs to be limited within a range lower than the maximum rated value of the speaker or sound amplifier so as to avoid resonance of the housing of the equipment and sound distortion of the speaker. However, with amplitude limitation of a sound signal at such a low value, the rated output value according to product specifications cannot be output.